Properties of a deep seismic waveguide measured with an optical fiber

Low-velocity zones located deep in the subsurface can act as seismic waveguides. Traditionally, their experimental observation has been limited by the practical challenges of in situ recording. We use a measurement technique in which optical fibers are turned into seismic sensors. The fiber is deployed along a horizontal well drilled inside a 15-m-thin shale formation at a depth of about 2 km. Owing to the high-resolution recording of the optical fiber, we can distinctly observe three previously elusive guided wave modes over a wide frequency range. As their propagation is primarily confined to the waveguide and strongly depends on its seismic properties, such guided waves hold tremendous potential for high-resolution imaging of deep low-velocity structures, such as fault zones, saline aquifers, and hydrocarbon reservoirs.